Optical head apparatus

Abstract: An optical head apparatus for a magneto-optical recording medium (110) including a light beam source (21), an objective lens (24), a separating optical system (23 - 27), and a photodetector (28). The objective lens (24) is provided for irradiating an output light beam from the light beam source (21) as a focused light beam to the surface of the signal recording surface of the magneto-optical recording medium (110). The separating optical system (23 - 27) separates the light beam radiated from the light beam source (21) from a reflected light beam reflected by the signal recording surface of the magneto-optical recording medium (110) via the objective lens (24). The photodetector (28) receives the reflected light beam thus separated by the separating optical system (23 - 27). The separating optical system (23 - 27) also includes an enhancement element (26) which apparently increases the Kerr rotation angle of the reflected light beam.

Description: BACKGROUND 1. Field of the Invention The present invention relates to an optical head. More particularly, the present invention relates to an optical head for recording signals onto an optical recording medium such as a magneto-optical recording medium, or for reading signals therefrom. 2. Background of the Invention In the case of an optical head for reading out information signals from a rewritable magneto-optical disc they can be fabricated, for example, into a structure illustrated in FIG. 1. Referring to FIG. 1, an optical head 1 includes a number of optical elements described below. A light beam, emitted from a semiconductor laser element 2 as a light beam source, is introduced into an objective lens 3, so that the light beam can be converged on the signal recording surface of the magneto-optical disc MO. The divergent light beam which is emitted from the semiconductor laser element 1 is converted into parallel light beams by a collimator lens 4. A beam splitter 5, which includes an optical prism (hereinafter referred to simply as a "prism") 51 adhered to an optical element 52 having its edges arranged in parallel with each other, separates the light beam into emitted for the semiconductor laser element 1 from a reflected light beam, i.e., a light beam reflected by the signal recording surface of the magneto-optical disc MO. In further detail, the beam splitter 5 has a multilayered dielectric layer in the boundary plane between the prism 51 and the optical element 52, that is, between the facet (edge plane) 5a of the optical element and the prism 51. This multilayered dielectric layer transmits the P-polarization component, but reflects the S-polarization component. A total reflection film which completely reflects the reflected light beam from the recording surface of the magneto-optical disc MO is provided on the other facet (edge plane) 5b of the optical element 52. The dielectric layer and the total reflection film are provided by means of vapor deposition and the like. A Wollaston prism 6 is provided to emit a plurality of beams based on the aforementioned reflected light beam from the signal recording surface. A Wollaston prism as disclosed, for example, in US-A-4,771,414 can be used in the present invention. The plurality of beams discharged from the Wollaston prism 6 are focused on the photodetector 9 by means of an imaging lens 7. A multi-purpose lens 8 is placed between the imaging lens 7 and the photodetector 9 to generate astigmatism for the detection of focusing error. Furthermore, since the multi-purpose lens 9 includes a concave plane on the output side, the optical path of the reflected light beam between the output end plane of the beam splitter 5 and the photodetector 9 can be shortened. The photodetector 9 has a plurality of photoreceptors for generating each of the following error signals; a focusing error signal and a tracking error signal. Then, signals according to the read-out signal from the information signal recorded on the signal recording surface of a magneto-optical disc MO are generated based on the detection signal provided by the photodetector 9. According to the optical head 1 according to the constitution above, the divergent P-polarization light beam emitted from a semiconductor laser element 1 is converted into parallel light beams by a collimator lens 4. The output light beam from the collimator lens 4 is then focused to one point on the signal recording surface of the magneto-optical disc MO by the objective lens 3 that is provided after the beam splitter 5. The light beams inclusive of the S-polarization component and the reflected light beam, which had been reflected by the signal recording surface of the magneto-optical disc MO, are again introduced to the beam splitter 51 via the objective lens 3. Then, the light beams are reflected by the boundary plane of the beam splitter and deflected their optical path polarization at an angle to 90°. They are further reflected completely and further deflected at the angle to 90° at the face 5b of the optical element 52. The Wollaston prism 6 emits a plurality of light beams according to the light beam irradiated by the beam splitter 5, so that the plurality of light beams which are provided to the photodetector 9 via a multi-purpose lens 8 might be focused on each of the photoreceptors of the photodetector 9. It should be noted that the light beam passed through the multi-purpose lens 8 has stigmatic aberration. Then, signals such as the signals read out from the magneto-optical disc MO, the focusing error signal, and the tracking error signal, are generated based on the detected signals from each of the photoreceptor planes of the photodetector 9. The signals recorded on the magneto-optical disc MO are read out based on the optical rotation of the beam according to Kerr effect. In the case of a read-only type optical discs, e.g., a so-called compact disc or an optical video disc, on the other hand, an optical head 10 for reading out information signals from these read-only discs can be fabricated, for example, in a constitution shown in FIG. 2. Referring to FIG. 2, a light beam emitted from a light beam source, i.e., a semiconductor laser 11, is divided into at least three light beams by a diffraction grating 15. The three light beams, i.e., the zeroth order diffraction light beam, the positive first order diffraction light beam, and the negative first order diffraction light beam thus generated by the grating 15 are deflected at the angle to 90° at a beam splitter 12 of a parallel plane type, and are further introduced into an objective lens 13. The beam splitter 12 for use herein is, in general, a non-polarizing beam splitter oriented at an angle of. 45° with respect to the optical axis of an objective lens 13. The objective lens 13 converges the light beam on one point of a signal recording surface of a rotation-driven optical disc D. More accurately, the zeroth order diffraction light beam is focused on the track of the signal recording surface, while the positive and the negative first order diffraction light beams are irradiated to the front and the back of the irradiation point of the zeroth order diffraction light beam in such a manner that it may be interposed between the irradiation points of the positive and the negative first order diffraction light beams. The light beam reflected by the signal recording surface of the optical disc D, i.e., the reflected light beam, is introduced again to the beam splitter 12 via the objective lens 13. Stigmatic aberration for the focus detection occurs on the reflected light beam during its transmission through the beam splitter 12. The output reflected light beam from the beam splitter 12 is then focused on the photoreceptor plane of the photodetector 16 via a concave lens 14. The use of a concave lens 14 not only shortens the optical path of the reflected light beam between the output edge plane of the beam splitter 12 and the photodetector 16 because the use thereof increases magnification, but also allows the reflected light beam to be focused on the photoreceptor plane of the photodetector 16 because the lens 14 can be moved to adjust its position along the optical axis of the objective lens 13. The photoreceptor plane of the photodetector 16 consists of a plurality of photoreceptor portions to accept focusing error signals and tracking error signals. As described in the foregoing, the optical head 10 of the constitution above is characterized in that a divergent light beam emitted from a semiconductor laser element 11 is introduced into the beam splitter 12 as it is via the diffraction grating 15. The light beam deflected at the angle to 90° and reflected by the beam splitter 12 is irradiated to focus on the signal recording surface of the optical disc D using the objective lens 13. The reflected light beam from the signal recording surface is introduced again to the beam splitter 12 via the objective lens 13. The light beam transmitted through the beam splitter 12 is focused on the photoreceptor plane of the photodetector via the concave lens 14. Thus, while a focusing error signal based on the output signal from the plurality of photoreceptor portions of the photodetector 16, which had received the zeroth diffraction light beam, and a signal from the read-out of the information recorded on the optical disc D are generated a tracking error signal is produced at the same time based on the output signal from a plurality of photoreceptor portions which had received the positive and the negative first order diffraction light beams. The optical heads described above with reference to FIGS. 1 and 2, however, still have the following problems to be solved. Referring to the optical head 1 illustrated in FIG. 1, the dielectric layer constituting the boundary plane of the beam splitter 5 is imparted with polarization characteristics to enhance the rotation angle of the polarization plane of the reflected light beam reflected from the signal recording surface of the magneto-optical disc MO. More specifically, the dielectric layer provided on the boundary plane of the beam splitter 5 is designed as such that it may yield a transmittance of the P-polarization component (TP) of 65 %, a reflectivity of the P-polarization component (RP) of 30 %, and a reflectivity of the S-polarization component (RS) of 95 % or higher. In this manner, the desired enhancement ratio can be obtained. However, the aforementioned dielectric layer, which changes the polarization state of the light beams and which is provided on the boundary plane of the aforementioned beam splitter 5, has an extremely large angle dependence. Accordingly, this dielectric layer is placed within parallel light beams as illustrated in FIG. 1. It then follows that at least the light beams passing the beam splitter 5 are converted into parallel light beams. It can therefore be seen from the foregoing that any system using a semiconductor laser element as the light beam source requires an additional optical element, specifically a collimator lens in the case illustrated in FIG. 1, for converting the divergent light beam irradiated from the semiconductor laser into parallel light beams. Furthermore, it is also necessary to include optical elements including, for example, a lens for focusing the light beams irradiated from the beam splitter. Accordingly, the optical head of the type above requires additional optical components to make a compact optical head unfeasible. On the other hand, the structure of the optical head 10 shown in FIG. 2 is simple. However, since a non-polarizing beam splitter is used, there is no enhancement in the rotation angle of the polarization plane of the reflected light beam. Accordingly, an optical head of this type yields a low CN (Carrier to Noise) ratio, and hence, is not suitable for reading out information signals recorded on a magneto-optical disc at high precision. Referring again to FIG. 1, the optical head 1 has an artificial quartz (SiO 2 ) as the Wollaston prism. The difference of indices of refraction Δn, as expressed by the equation below, of this Wollaston prism upon using a light beam source with a wavelength λ of 780 nm is about 0.0089: Δn ≡ n e - n o where, n e is the index of refraction of an extraordinary light beam, and n o is the same of an ordinary light beam. In the case of a Wollaston prism using an artificial quartz, the separating angle between the extraordinary light beam and the ordinary light beam is about one degree when the prism is adhered at an angle of 45° with respect to the crystallographic orientation. Thus again, if a compact optical head were to be implemented by the constitution above, the Wollaston prism should be provided at a thickness of at least 4 mm. Accordingly, a compact head cannot be accomplished as long as it is based on this constitution. EP-A-0 141 679, on which the preamble of claim 1 is based, discloses an optical head apparatus for a magneto-optical recording medium, with a light beam source for emitting a light beam, an objective lens which focuses the light beam emitted from said light beam source to the recording surface of a magneto-optical recording medium, a separating optical system comprising a beam splitter for separating the beam into a light beam emitted from said light beam source and a reflected light beam reflected from the recording surface of the magneto-optical recording medium via said objective lens, and a photodetector for receiving said reflected light beam reflected from the recording surface, being separated by said separating optical system, whereby in said separating optical system a Kerr rotation angle of the reflected light beam is increases. Thereby, the beam splitter is a polarizing beam splitter which at the same time increases the Kerr rotation angle of the reflected light beam. The disadvantage of this prior art optical head apparatus is the increase of the Kerr rotation angle of the reflected light beam depends on optical conditions of the optical system, since the functions of the Kerr rotation angle increase and the beam splitting are combined in the polarizing beam splitter. The influence of optical conditions may have adverse effects on the CN ratio and the read out and reproduction characteristics. The object of the present invention is therefore to provide an optical head apparatus according to the preamble of claim 1, in which the influence of optical conditions is as low as possible, so that a large CN ratio and favourable readout and reproduction characteristics are achieved. This object is achieved by an optical head apparatus according to claim 1. Advantageous features are specified in the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more readily understood with reference to the accompanying drawings, wherein: FIG. 1 shows a schematically drawn structure of a prior art optical head; FIG. 2 shows a schematically drawn structure of another prior art optical head; FIG. 3 shows a schematically drawn structure of an optical head according to an embodiment of the present invention; FIG. 4 shows the relation between the Kerr rotation angles for an incident beam and a reflected light beam in an enhancement element according to the present invention; FIGS. 5 (A) to 5 (C) each schematically show modified types of the enhancement element according to the present invention, where FIG. 5 (A) shows a cubic enhancement element, FIG. 5 (B) shows a planar enhancement element, and FIG. 5 (C) shows an enhancement element including a pair of parallel planar plates; DESCRIPTION OF THE INVENTION The present invention is illustrated in greater detail referring to the drawings. In the non-limiting examples below, reference is made particularly to embodiments in which the optical heads according to the present invention are applied to magneto-optical recording medium such as a magneto-optical disc. It should be understood, however, that the present invention is not to be construed as being limited thereto. Referring to FIG. 3, an optical head according to an embodiment of the present invention is described. An optical head 20 illustrated in FIG. 3 has a number of optical elements described below. A light beam, emitted from a semiconductor laser element 21 which functions as a light beam source, is introduced as it is in the state of a divergent light beam into a beam splitter 23 via a diffraction grating 22. The light beam emitted from the semiconductor laser 21 is divided into at least three light beams by the diffraction grating 22, and irradiated to the beam splitter 23. The three light beams, i.e., the zeroth order diffraction light beam, the positive first order diffraction light beam, and the negative first order diffraction light beam thus generated by the grating 22, which are shown by a single light beam in the drawing for simplification, are deflected at the angle to 90° and reflected at the same time by the beam splitter 23. The beam splitter 23 used herein is a non-polarizing beam splitter of a parallel plane type, which includes parallel planes made of a material such as a glass and a multilayered film formed on the incident plane 23a which receives the light beam from the semiconductor laser element 21 and the reflected light beam reflected by the magneto-optical disc MO. This multilayered film has no polarizing effects. The beam splitter 23 is oriented at an angle of 45° with respect to the optical axis passing through the objective lens. The light beam reflected and its deflected at the angle to 90° on encountering the beam splitter 22 is further introduced into an objective lens 24. The objective lens 24 converges the light beam on one point of a signal recording surface of a magneto-optical disc MO. More accurately, the zeroth order diffraction light beam is focused on the track of the signal recording surface, while the positive and the negative first order diffraction light beams are irradiated to the front and the back of the irradiation point of the zeroth order diffraction light beam in such a manner that it may be interposed between the irradiation points of the positive and the negative first order diffraction light beams. The reflected light beam, i.e., the reflected light beam generated by the signal recording surface of the magneto-optical disc MO have their polarization planes rotated by Kerr effect. Accordingly, the reflected light beam is introduced again to the beam splitter 23 via the objective lens 24. Stigmatic aberration for the focus detection is generated on the reflected light beam during its transmission through the beam splitter 23. A concave lens 25 is provided as an intermediate lens. The concave lens 25 functions principally for shortening the optical path of the reflected light beam between the output edge plane of the beam splitter 23 and the photodetector to be described hereinafter. Furthermore, this concave lens 25 allows the reflected light beam to be focused on the photoreceptor plane of the photodetector while maintaining the reflected light beam in a focused state and the focusing error signal null, because the concave lens 25 can be moved to finely adjust its position along the optical axis of the objective lens 24. An enhancement element 26 includes a multilayered dielectric designed as such that it may yield a transmittance of the S-polarization component (TS) of about 30 % and a transmittance of the P-polarization component (TP) of approximately 100 %. The multilayered dielectric is provided by means of deposition such as vapor deposition and sputtering, on the position corresponding to the boundary plane of the either of a pair of prisms constituting the enhancement element 26. A cubic-shaped enhancement element 26 can be implemented by adhering the pair of prisms. The Kerr rotation angle  2 of a light beam passed through the enhancement element 26 produced in this manner is illustrated in FIG. 4, and can be expressed by the following equation: 2 = ( TP TS ) 1/2 · 1 where,  1 represents the Kerr rotation angle of the incident light beam. The light beam passed through the enhancement element 26 is introduced into a Wollaston prism 27. The Wollaston prism 27 generates a plurality of light beam according to the light beam passed through the enhancement element 26. A photodetector 28 having a plurality of photoreceptor portions is provided to receive, among the plurality of light beams irradiated from the Wollaston prism, at least the light beams necessary for generating error signals such as a focusing error signal and a tracking error signal, or for generating the read-out signals of the magneto-optical disc MO. Thus, in the optical head 20 described in the foregoing, the light beam emitted from the semiconductor laser element 21 is introduced into the beam splitter 23 via the diffraction grating 22. The light beam reflected and deflected at the angle to 90° by the beam splitter 23 is focused on one point of a signal recording surface of a magneto-optical disc MO via the objective lens 24. The light beam reflected by the signal recording surface of the magneto-optical disc MO, i.e., the reflected light beam, has its rotation angles of the polarization plane rotated by Kerr effect, and re-enter into the beam splitter 23 via the objective lens 24. The reflected light beam irradiated to the beam splitter 23 is transmitted through the beam splitter 23, and is received by the photodetector 28 via the concave lens 25, the enhancement element 26, and the Wollaston prism 27. Among the output signals from the plural photoreceptor planes of the photodetector 28, the output signal from the photoreceptor plane which received the zeroth order diffraction beam is used to generate the focusing error signals, whereas the output signals from the respective photoreceptor planes that received the positive and the negative first order diffraction beam are used in the generation of tracking error signals. The signals generated upon reading out the information signals recorded on the magneto-optical disc MO are generated by using two light beams among the three light beams generated by the Wollaston prism from the zeroth order diffraction light beam. An apparent increase of Kerr rotation angle is perceived when the reflected light beam passes through the enhancement element 26. This signifies that the three diffraction beams, i.e., the zeroth order diffraction beam and the positive and negative first order diffraction beams, are each further separated into P-polarization, S-polarization, and (P+S) polarization beams attributed to the birefringence induced by the Wollaston prism 27. Thus, nine light beams in total are generated from the Wollaston prism 27. The separation angle in this case amounts to a relatively large value attributed to Kerr effect. The nine light beams are generated by the Wollaston prism 27, but at least five of them, i.e., three zeroth order diffraction beams and two first order diffraction beams, are used to detect and generate, a focusing error signal, a tracking error signal, and a read-out signal from the magneto-optical disc MO, at high precision. In addition to the cubic-shaped enhancement element 26A shown in FIG. 5 (A), other types of an enhancement element can be used. Examples of the enhancement elements are given in FIGS. 5 (B) and 5 (C). Referring to FIG. 5 (B), an enhancement element 26B includes parallel plates made of a glass, etc., and a multilayered dielectric is formed on one of the planes of the plates, i.e., the incident plane of the reflected light beam and also of the light beam irradiated from the light beam source. The multilayered dielectric is formed on the plate by a deposition process such as vapor deposition and sputtering, and is designed as such that it may yield a transmittance of the P-polarization component (TP) of more than 90 %, a transmittance of the S-polarization component (TS) of 30 %, and a reflectivity of the S-polarization component (RS) of 65 %. An enhancement element 26C shown in FIG. 5 (C) differs from the enhancement element 26A of FIG. 5 (A) which includes a pair of prisms adhered together. That is, the enhancement element 26C includes a pair of parallel plated adhered together. In the same manner as in the case of the enhancement element shown in FIG. 5(A), a multilayered dielectric is provided between the pair of parallel plates by a deposition process such as vapor deposition or sputtering, in such a manner that it may yield a transmittance of the P-polarization component (TP) of more than 95 %, a transmittance of the S-polarization component (TS) of 30 %, and a reflectivity of the S-polarization component (RS) of 65 %. The cubic-shaped enhancement element 26A differs from an ordinary polarization beam splitter in that it need not control the phase difference between the P-polarization component and the S-polarization component. Similar to the case of the enhancement element 26A, the enhancement element 26C yields a sufficient angle dependence of transmitted light beam, and a large Kerr rotation angle. Yet, the production cost thereof can be reduced as compared with the cubic-shaped enhancement element 26A. The enhancement element 26B yields a transmitted light beam having a relatively low angle dependence, however, the CN ratio is sufficiently high, and the fabrication cost can be reduced as compared with the enhancement element 26C because of its simple structure.

Claims: An optical head apparatus for a magneto-optical recording medium, said apparatus comprising: a light beam source (21) for emitting a light beam; an objective lens (24) which focuses the light beam emitted from said light beam source (21) to the recording surface of a magneto-optical recording medium (110); a separating optical system (23-27) comprising a beam splitter (23) for separating the beam into a light beam emitted from said light beam source (21) and a reflected light beam reflected from the recording surface of the magneto-optical recording medium (110) via said objective lens (24), and a photodetector (28) for receiving said reflected light beam reflected from the recording surface (110), being separated by said separating optical system (23-27), whereby in said separating optical system (23-27) a Kerr rotation angle of the reflected light beam is increased, characterized by an enhancement element (26A; 26B; 26C) placed in the optical path between said beam splitter (23) and said photodetector (28) for increasing the Kerr rotation angle of the reflected light beam. An optical head apparatus according to claim 1, characterized in, that said enhancement element (26A; 26B; 26C) comprises a deposited multilayered dielectric. An optical head apparatus according to claim 2, characterized in, that said enhancement element (26A) comprises a pair of prisms adhered together. An optical head apparatus according to claim 2, characterized in, that said enhancement element (26B) comprises parallel plates. An optical head apparatus according to claim 2, characterized in, that said enhancement element (26C) comprises a pair of parallel plates adhered together. An optical head apparatus according to any one of claims 1 to 5, characterized in, that said separating optical system (23-27) comprises a Wollaston prism made of lithium niobate, said Wollaston prism being placed between said enhancement element (26A; 26B; 26C) and said photodetector (28).

Inventors: ANDO NOBUHIKO, HINENO SATOSHI, ANDO, NOBUHIKO, HINENO, SATOSHI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1994
Main classification: G11B11
IPC codes: G11B7, G11B11